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  • 1

    Deactivation Behaviour of Supported Gold Palladium Nanoalloy

    Catalysts during the Selective Oxidation of Benzyl Alcohol in a Micro-

    Packed Bed Reactor

    Noor Al-Rifai1,5, Peter Miedziak2, Moataz Morad2,3, Meenakshisundaram Sankar2, Conor Waldron1,

    Stefano Cattaneo2, Enhong Cao1, Samuel Pattisson2, David Morgan2, Donald Bethell4, Graham

    Hutchings2 and Asterios Gavriilidis1*

    1Department of Chemical Engineering, University College London, Torrington Place, London WC1E

    7JE, UK

    2Cardiff Catalysis Institute, School of Chemistry, Cardiff University, CF10 3AT, UK

    3Chemistry Department, Faculty of Science, Umm Al-Qura University, PO. Box 21955, 9264 Makkah,

    Saudi Arabia

    4Department of Chemistry, University of Liverpool, Crown Street, Liverpool, L69 7ZD, UK

    5Coty Inc., Luxury Fragrances R&D, Product Development, HFC Prestige, Egham, TW20 9NW, UK

    Corresponding author: [email protected]

    mailto:[email protected]

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    Abstract

    Highly active, supported Au-Pd catalysts have been tested for catalysing benzyl alcohol oxidation in a

    silicon-glass micro-packed bed reactor. The effects of Au-Pd composition and anion content during catalyst

    preparation on catalyst deactivation were studied, and a relationship between the deactivation rate and the

    amount of Cl- and Au used in the catalyst formulation was found. Whilst Au aids in enhancing the selectivity

    to the desired product and the Cl- ions help the formation of uniform 1-2 nm nanoparticles, higher amounts

    of Au and Cl- become detrimental to the catalyst stability once a certain amount is exceeded. Loss of small

    (1-2 nm) metal nanoparticles was evident in all catalysts studied, accompanied by agglomeration and the

    formation of larger >10 nm particles. A secondary deactivation mechanism characterised by the formation

    of an amorphous surface film was observed via TEM in catalysts with high Cl- and Au and was confirmed

    by the detection of carbon species on the catalyst surface using Raman Spectroscopy.

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    1. Introduction

    The selective oxidation of alcohols to carbonyl compounds is an important process due to the value of the

    aldehyde products as intermediates in the manufacture of pharmaceuticals, fragrances and fine chemicals

    [1]. Conventionally, these carbonyl compounds are produced via methods that are harmful to the

    environment, utilising stoichiometric oxidants such as permanganates or chromates that generate copious

    amounts of heavy metal containing wastes. Catalytic oxidation using environmentally benign oxidants like

    O2, H2O2 or air has received considerable attention due to water being the main by-product of these

    reactions. Gold-palladium nanoalloy catalysts have been demonstrated to be particularly active for these

    transformations and superior in performance to their monometallic counterparts [2;3]. The catalyst

    preparation method plays a central role in determining the size, composition and nanostructure of these

    nanoparticles, with smaller particles being the most catalytically active [4].

    Trade-offs exist between complexity of preparation method and the catalytic activity of the resulting

    catalyst. The simplest method of preparing Au-Pd nanoalloy catalysts is conventional impregnation,

    however, this method produces nanoparticles with a wide particle size distribution ranging from 1 to 10

    nm, together with large >10 nm particles. Catalysts prepared by sol-immobilisation – a method that uses

    stabiliser ligands to control the particle size and avoid the formation of catalytically inactive larger particles

    – were found to have high catalytic activity, but the disadvantage of being unstable [5]. Recent studies on

    the use of an excess anion modified impregnation (MIm) method in the preparation of supported gold-

    palladium catalysts have shown that through the addition of excess chloride ions to the metal precursors

    during the impregnation stage, a catalyst that possesses a very tight particle size distribution can be

    produced [6;7]. The improvement in activity is postulated to be due to the combination of particle size

    control, elimination of size dependent compositional variation, and the random alloy nanostructure [6].

    One of the major challenges in commercialising promising catalysts is ensuring stability under industrially

    relevant conditions [8]. Typically, the stability of these catalysts is studied in a batch reactor using a

    “recover and re-use” strategy, a procedure that is not representative of typical industrial operation. A more

    effective method for testing the catalyst stability is through the use of a flow reactor, where the stability of

    the catalyst can be monitored with time on stream. The small inventories of catalyst and reactants required

    in microreactors gives them competitive edge as laboratory tools for this kind of studies. Rapid

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    manipulation of reaction conditions, fast response times, and precise control over the hydrodynamic

    environment have increased their use in reaction kinetic studies, in-situ spectroscopic characterisation, and

    reaction optimisation, making them ideal candidates for testing of catalytic stability [9].

    The deactivation of solid catalysts in liquid phase reactions is usually caused by physical, thermal, or

    chemical changes of the catalyst [10]. Four main modes of deactivation have been identified in Pt-group

    metal catalysed reactions: (1) change of oxidation state of the active metal site (“over-oxidation”), (2)

    irreversible adsorption of products or polymeric species, and (3) loss of metal surface area by leaching or

    (4) sintering [11;12]. The inhibitory effect of chlorine when used in a metal precursor has been investigated

    by others, but primarily for gas phase reactions [13;14]. Peri et al. [13] raised the possibility that the

    activation phenomenon (the increase in catalytic activity with time on stream at start-up) in catalysts

    prepared from chlorinated precursors, could be due to the slow removal of chlorine with time. The negative

    effect of residual chlorine on the catalyst has been attributed to several causes, including partial blockage

    of metal particles by chlorine [15] and the generation of metal oxychloride species that have higher

    reduction temperatures and lead to a less active oxidation site [16]. A mechanism for the mobility of

    chloride ions during reduction and reaction has been presented elsewhere [15]; the reduction of the catalyst

    has been postulated to cause mobility of the chloride ions from the surface of the metal particles to the

    interior of the particles and on the support. The introduction of the oxidant is then hypothesised to cause

    movement of the chlorine to the metal particle surface again. The use of H2 + H2O treatment has been

    shown to favour the definitive chlorine elimination from the catalyst (i.e. H2O reacts with the Cl to produce

    HCl).

    The objective of this work is to investigate the stability of supported 1%AuPd/TiO2 catalyst prepared by

    impregnation methodologies during the aerobic selective oxidation of benzyl alcohol in a micro packed bed

    reactor. Three catalyst preparation methods are investigated: conventional impregnation and two types of

    modified impregnation methods, where an excess of anion (chloride ions) during the wet impregnation

    stage is used. The influence of Au and Cl- ion contents on catalyst stability is studied, and a possible

    deactivation mechanism is presented, aided by the following characterisation techniques: scanning electron

    microscopy (SEM), transmission electron microscopy (TEM), x-ray photoelectron spectroscopy (XPS),

    atomic emission spectroscopy (AES), and Raman spectroscopy.

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    1.1 Benzyl Alcohol Oxidation on Supported Au-Pd Catalysts

    A generalised reaction scheme for the oxidation of benzyl alcohol on Au-Pd catalysts is presented in Figure

    1. In the presence of oxygen, the oxidation of benzyl alcohol leads in the first instance to the formation of

    benzaldehyde, which further oxidises to benzoic acid [17]. The formation of benzyl benzoate is through an

    esterification reaction between the initial alcohol and the (i) generated aldehyde (in the presence of oxygen)

    or (ii) the generated benzoic acid [18]. Typically, benzoic acid, benzyl benzoate, and dibenzyl ether are

    produced in much smaller quantities (

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    2 Materials & Methods

    2.1 Catalyst Preparation Methods

    Two main catalyst preparation methods are used in this study: Conventional Impregnation (CIm) and

    Modified Impregnation (MIm). Within the modified impregnation category, there exists two types: Modified

    Impregnation Constant Cl (MIm,const) and Modified Impregnation Varying Cl (MIm,vary). The catalyst

    preparation methods for each of these catalysts will be presented next.

    2.1.1 Conventional Impregnation, CIm

    The Conventional Impregnation (CIm) preparation procedure for supported Au-Pd catalysts relies on a wet-

    impregnation method, which has been reported in previous articles [4]. In a typical synthesis, an aqueous

    mixture of the m

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